universe Review Quantum String Cosmology Maurizio Gasperini 1,2 1 Dipartimento di Fisica, Università di Bari, Via G. Amendola 173, 70126 Bari, Italy; [email protected] 2 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Via E. Orabona 4, 70125 Bari, Italy Abstract: We present a short review of possible applications of the Wheeler-De Witt equation to cosmological models based on the low-energy string effective action, and characterised by an initial regime of asymptotically flat, low energy, weak coupling evolution. Considering in particular a class of duality-related (but classically disconnected) background solutions, we shall discuss the possibility of quantum transitions between the phases of pre-big bang and post-big bang evolution. We will show that it is possible, in such a context, to represent the birth of our Universe as a quantum process of tunneling or “anti-tunneling” from an initial state asymptotically approaching the string perturbative vacuum. Keywords: string cosmology; quantum cosmology; Wheeler-DeWitt equation 1. Introduction In the standard cosmological context the Universe is expected to emerge from the big bang singularity and to evolve initially through a phase of very high curvature and density, well inside the quantum gravity regime. Quantum cosmology, in that context, turns out to be a quite appropriate formalism to describe the “birth of our Universe”, possibly in a state approaching the de Sitter geometric configuration typical of inflation (see, e.g., [1] for a review). In the context of string cosmology, in contrast, there are scenarios where the Universe Citation: Gasperini, M. Quantum emerges from a state satisfying the postulate of “asymptotic past triviality” [2] (see [3] String Cosmology. Universe 2021, 7, for a recent discussion): in that case the initial phase is classical, with a curvature and a 14. https://doi.org/10.3390/ density very small in string (or Planck) units. Even in that case, however, the transition to universe7010014 the decelerated radiation-dominated evolution, typical of standard cosmology, is expected to occur after crossing a regime of very high-curvature and strong coupling. The birth of Received: 26 November 2020 our Universe, regarded as the beginning of the standard cosmological state, corresponds in Accepted: 9 January 2021 Published: 12 January 2021 that case to the transition (or “bounce”) from the phase of growing to decreasing curvature, and even in that case can be described by using quantum cosmology methods, like for a Publisher’s Note: MDPI stays neu- Universe emerging from an initial singularity. tral with regard to jurisdictional clai- There is, however, a crucial difference between a quantum description of the “big ms in published maps and institutio- bang” and of the “big bounce”: indeed, the bounce is preceded by a long period of nal affiliations. low-energy, classical evolution, while the standard big bang picture implies that the space- time dynamics suddenly ends at the singularity, with no classical description at previous epochs (actually, there are no “previous” epochs, as the time coordinate itself ends at the singularity). In that context the initial state of the Universe is unknown, and has to be Copyright: © 2021 by the author. Li- fixed through some ad hoc prescription: hence, different choices for the initial boundary censee MDPI, Basel, Switzerland. conditions are in principle allowed [4–9], leading in general to different quantum pictures This article is an open access article for the very early cosmological evolution. Such an approach, based in particular on “no- distributed under the terms and con- boundary” initial conditions, has been recently applied also to the ekpyrotic scenario [10], ditions of the Creative Commons At- leading to the production of ekpyrotic instantons [11,12]. In the class of string cosmology tribution (CC BY) license (https:// models considered in this paper, in contrast, the initial state is uniquely determined by creativecommons.org/licenses/by/ 4.0/). a fixed choice of pre-big bang (or pre-bounce) evolution (see, e.g., [13–17]), which starts Universe 2021, 7, 14. https://doi.org/10.3390/universe7010014 https://www.mdpi.com/journal/universe Universe 2021, 7, 14 2 of 14 asymptotically from the string perturbative vacuum and which, in this way, unambiguously determines the initial “wave function” of the Universe and the subsequent transition probabilities. In this paper we report the results of previous works, based on the study of the Wheeler–De Witt (WDW) equation [18,19] in the “minisuperspace” associated with a class of cosmological backgrounds compatible with the dynamics the low-energy string effective action [20–26]. It is possible, in such a context, to obtain a non-vanishing transition probability between two different geometrical configurations—in particular, from a pre- big bang to a post-big bang state—even if they are classically disconnected by a space- time singularity. There is no need, to this purpose, of adding higher-order string-theory contributions (like a0 and loop corrections) to the WDW equation, except those possibly encoded into an effective (non-local) dilaton potential (but see [27,28] for high-curvature contributions to the WDW equation). It will be shown, also, that there are no problems of operator ordering in the WDW equation, as the ordering is automatically fixed by the duality symmetry of the effective action. Other possible problems—of conceptual nature and typical of the WDW approach to quantum cosmology—however, remain, like the the validity of a probabilistic interpretation of the wave function [29], the existence and the possible meaning of a semiclassical limit [30], the unambiguous identification of a time-like coordinate in superspace (see however [23], and see [31] for a recent discussion). Let us stress that this review is dedicated in particular to string cosmology back- grounds of the pre-big bang type, and limited to a class of spatially homogeneous geome- tries. It should be recalled, however, that there are other important works in a quantum cosmology context which are also directly (or indirectly) related to the string effective action, and which are applied to more general classes of background geometries not neces- sarily characterised by spatial Abelian isometries, and not necessarily emerging from the string vacuum. We should mention, in particular, the quantum cosmology results for the bosonic sector of the heterotic string with Bianchi-type IX geometry [32] and Bianchi class A ge- ometry [33]; solutions for the WDW wave function with quadratic and cubic curvature corrections [34] (typical of f (R) models of gravity), describing a phase of conventional inflation; two-dimensional models of dilaton quantum cosmology and their supersymmet- ric extension [35]; WDW equation for a class of scalar-tensor theories of gravity with a generalised form of scale-factor duality invariance [36,37]. We think that discussing those (and related) works should deserve by itself a separate review paper. This paper is organized as follows. In Section2 we present the explicit form of the WDW equation following from the low-energy string effective action, for homogeneous backgrounds with d Abelian spatial isometries, and show that it is free from operator- ordering ambiguities thanks to its intrinsic O(d, d) symmetry. In Section3, working in the simple two-dimensional minisuperspace associated with a class of exact gravi-dilaton solutions of the string cosmology equations, we discuss the scattering of the WDW wave function induced by the presence of a generic dilaton potential. In Section4 we show that an appropriate quantum reflection of the wave function can be physically interpreted as representing the birth of our Universe as a process of tunnelling from the string perturbative vacuum. Similarly, in Section5, we show that the parametric amplification of the WDW wave function can describe the birth of our Universe as a process of “anti-tunnelling” from the string perturbative vacuum. Section6 is finally devoted to a few conclusive remarks. 2. The Wheeler-De Witt Equation for the Low-Energy String Effective Action In a quantum cosmology context the Universe is described by a wave function evolving in the so-called superspace and governed by the WDW equation [18,19], in much the same way as in ordinary quantum mechanics a particle is described by a wave function evolving in Hilbert space [38], governed by the Schrodinger equation. Each point of superspace corresponds to a possible geometric configuration of the space-like sections of our cosmological space-time, and the propagation of the WDW wave function through this Universe 2021, 7, 14 3 of 14 manifold describes the quantum dynamics of the cosmological geometry (thus providing, in particular, the transition probabilities between different geometric states). The WDW equation, which implements the Hamiltonian constraint H = 0 in the superspace of the chosen cosmological scenario, has to be obtained, in our context, from the appropriate string effective action. Let us consider, to this purpose, the low-energy, tree- level, (d + 1)-dimensional (super)string effective action, which can be written as [39–41] 1 Z p 1 = d+1 f + ¶ f¶mf mna + , (1) S d 1 d x g e− R m Hmna H V − 2ls − − − 12 where f is the dilaton [42], Hmna = ¶mBna + ¶nBam + ¶aBmn is the field strength of the NS-NS two form B = B (also called Kalb-Ramond axion), and l (a )1/2 is the mn − nm s ≡ 0 fundamental string length parameter. We have also added a possible non-trivial dilaton potential V(f). For the purpose of this paper it will be enough to consider a class of homogeneous backgrounds with d Abelian spatial isometries and spatial sections of finite volume, i.e., R d ( d xp g) = < ¥. In the synchronous frame where g = 1, g = 0 = B , and − t const 00 0i 0i where the fields are independent of all space-like coordinates xi (i, j = 1, .., d), the above action can then be rewritten as follows [43,44]: Z ls f ˙ 2 1 1 S = dt L(f, M), L = e− (f) + Tr M˙ (M− )˙+ V .